Anomalous elastic properties of coesite at high pressure and implications for the upper mantle X-discontinuity

Compressional and shear wave velocities of coesite have been measured using ultrasonic interferometry in a multi-anvil apparatus up to 12.6 GPa at room temperature for the first time. While the P wave velocity increases continuously with pressure, the S wave exhibits an anomalous softening and the velocity decreases continuously with pressure. Finite strain analysis of the data yielded K_S0=103.6(4)GPa, G₀=61.6(2)GPa and K0'=2.9(1), G0'=0.3(1) for the bulk and shear moduli and their pressure derivatives, respectively. The anomalous elastic behavior of coesite results in large velocity and impedance contrasts across the coesite-stishovite transition, reaching ~39% and ~48% for P and S wave velocity contrasts, and ~70% and 78% for P and S wave impedance contrasts, respectively, at pressure ~8 GPa, with P and S wave velocity perturbations showing no apparent dependence on depths (i.e., dln V_(PorS)/dh~0) within 8-12 GPa. These unusually large contrasts and depth independent characteristics render the transition between the two silica polymorphs one of the most plausible candidates for the cause of the seismically observed X-discontinuity. The current P and S wave velocity perturbation dependences on the SiO₂ content, d(ln V_P)/d(SiO₂)~0.43(wt%)^-1 and d(ln V_S)/d(SiO₂)~0.60(wt%)^-1, can serve as a geophysical probe to track ancient subducted eclogite materials to gain insights on the geodynamics of the mantle.